Multiple mechanisms operant in MS culminate ultimately in the generation of a highly oxidative microenvironment that is responsible for tissue injury and neurodegeneration44,45
. The data presented here add to a growing body of literature that strongly support the clinical development of the family of natural and synthetic triterpenoids as chemopreventive and therapeutic agents in the context of neuro-inflammatory and neurodegenerative diseases, and specifically in the context of MS. While neuroprotective effects of administered exogenous antioxidants have been reported in EAE5,46
, pursuit of strategies that employ newer agents such as synthetic triterpenoids to induce endogenous enzymatic antioxidants should be a more efficient means to inhibit the detrimental effects of ROS and thereby serve as an effective form of chemoprevention for MS.
Synthetic oleanane triterpenoids have emerged as a potent class of pharmacologic agents capable of strongly inducing the Nrf2-ARE axis, and this mechanism has been identified as playing a principal role in the neuroprotective effects of CDDO derivatives in a variety of models of neurodegenerative disorders27
. For example, in the murine SOD-1 model of Amyotrophic Lateral Sclerosis (ALS)47
, exposure to CDDO-EA and CDDO-TFEA results in nuclear translocation of Nrf2 in cells expressing the SOD-1 mutant, and suppressed disease progression whether treatment was initiated in asymptomatic mice or at the onset of clinical manifestations. While the clinical response observed in the EAE model may be attributed to effects on Nrf2 expression and function, triterpenoids are multifunctional agents and it is quite likely that multiple targets participate in conferring protection in the setting of a complex neurodegenerative process like MS. For example, a role for Stat3 not only in the genesis of Th17 cells but also in the development of EAE has been defined48,49,50
. Our laboratory and others have shown that Stat3 is a direct target of the synthetic triterpenoids28,51
. Our demonstration that triterpenoid exposure following induction of EAE significantly represses both IL-17 mRNA and protein expression suggests that this mechanism may also contribute to the overall clinical response to triterpenoids in the EAE model.
It is noteworthy that the link between an induction of Hmox-1 and repression of the development of a Th17 reponse has also been demonstrated for other activators of Hmox-1 in preclinical models of EAE and inflammatory bowel disease (IBD). Hemin, a potent inducer of Hmox-1 and activator of Nrf252
, suppresses the development of IBD in mice exposed to dextran sodium sulfate (DSS) and this effect is also associated with a significant reduction in the production of IL-1753
. Similarly, intraperitoneal administration of erythropoietin to MOG-EAE mice significantly reduced severity of disease in a manner that correlated with potent upregulation of Hmox-1 in the CNS and spleen and with a reduction of Th-17 T cell subsets54
. Finally, the observation that IL-17 might act to directly suppress the expression of Hmox-1 in the context of an inflammatory response argues that the balance between these two factors may in fact be an important determinant of disease expression. Thus the unique capacity of triterpenoids to simultaneously repress IL-17 expression through inhibition of Stat3 and to activate HO-1 through Nrf-2 suggests their utility for treatment of MS.
It is intriguing to find the induction of both 11β-hydroxysteroid dehydrogenase type 1 (11bHSD1) and of prostaglandin D2
synthase (PGDS) in the CNS in mice exposed to CDDO-TFEA. It has been suggested that a local deficit of cortisol in the CNS may underlie the relapsing-remitting nature of MS, perhaps secondary to poor local activation of cortisone via 11β-hydroxysteroid dehydrogenase type 1 (11bHSD1)55
. MS patients have been found to maintain normal cortisol concentrations in serum yet exhibit lowered cortisol levels in the CSF during acute relapses. Although 11bHSD1 was not found to be expressed within active plaques, the significant induction in myelin-loaded macrophages in vitro
suggests that production of this enzyme by infiltrating macrophages may contribute to the self-limiting nature of MS lesion development. Similarly, a reduction in the intrathecal synthesis of prostaglandin D2
synthase (PGDS) has been documented during acute inflammatory demyelination56
, and enhanced biosynthesis of prostaglandin (PG)D2
and subsequent formation of 15-deoxy-Δ-PGJ2
has been implicated in the resolution of inflammation57
. It is intriguing that 15-PGJ2
has been shown to activate Nrf2 signaling and to induce Hmox-1 in a manner similar to CDDO58
thus demonstrating the capacity to contribute to the endogenous anti-oxidant response and raising the question whether the observed clinical response to triterpenoids shown here might reflect their capacity to mimic many of the functions of this endogenous anti-inflammatory protein.
Lastly, our data in the model of LPC-induced demyelination suggest that the efficacy of triterpenoids in the context of EAE is linked to a capacity to regulate astrocyte activation and to preserve CNS myelin. Nrf-2-/- mice develop a spontaneous progressive leukoencephalopathy, with vacuolar degeneration that is linked to myelin unwinding, intramyelinic cysts and oxidative damage to myelin10
. Similarly, the observation that the anti-oxidant fumaric acid (FA) can preserve myelin and axonal density has also been linked to the capacity of FA and its esters to regulate Nrf-2 activity, and may be a mechanism contributing to the observed reduction of gadolinium enhancing lesions in MS patients treated with dimethylfumarate59
. Indeed, the beneficial effects of dimethylfumarate for axon preservation and astrocyte activation in the MOG-EAE model are lost in mice deficient in Nrf-2 gene expression60
In conclusion, the data present here demonstrate the efficacy of multifunctional triterpenoids in suppressing clinical and pathologic features of disease in preclinical models of MS. These small molecules have a unique capacity to simultaneously target intermediates that drive production of disease-inducing cytokines, such as IL-17, while inducing neuroprotective pathways activated through Nrf-2. Thus, synthetic triterpenoids may serve as both therapeutic and chemopreventive agents in the context of neurodegenerative and inflammatory diseases affecting the CNS.